1. Field of the Invention
The present invention relates generally to electric propulsion units for recreational watercraft. More specifically the present invention relates to propulsion units which employ brushless DC motors and more particularly the utilization of switched reluctance motors in such propulsion units.
2. Description of the Related Art
Recreational watercraft are extremely popular for a variety of uses. Some of the more typical uses may include water skiing, other suitable towing activities, fishing, and pleasure riding, just to name a few. All of these activities typically require the ability of a propulsion system to move the watercraft through the water by providing an adequate amount of thrust.
Many watercraft rely on an outboard, or inboard/outboard motor as a primary propulsion device. Such motors generally include an internal combustion engine to supply the required thrust. In watercraft outfitted for fishing purposes, a secondary propulsion system is often employed by the watercraft. These secondary propulsion systems are commonly known as trolling motors or electric outboards. Trolling motors are typically light weight electric motors drivingly coupled to a prop for propelling a fishing boat. Because of the relatively small size and quiet operation of such motors, they are often used to propel the boat into remote and shallow regions of a body of water. The motors may be operated and controlled without diverting the user from fishing in many instances. Such propulsion systems are often connected to an operator input device, such as a foot pedal. The operator will utilize the foot pedal to control the thrust and direction of the trolling motor. Depending upon the design, the electric motor may be directly adjacent to the prop, and thus submersed in the body of water. In other designs, the electric motor may be coupled to the prop by a shaft or other driving component, allowing the electric motor to remain out of the water during operation.
The electric motors used for such service today are typically brush-type or commutated DC motors. In one such design, the motor has magnets located around the inner periphery of the motor housing. A rotating member, known as the armature, is located in the interior of the housing and concentric with the magnets. The armature is composed of loops or windings of wire connected to wedge shaped segments known as commutator bars. Contact components or brushes, typically made of copper and carbon, contact the commutator bars to conduct electrical energy to the rotor windings. As current flows through the windings, a magnetic field is generated which interacts with that of the magnets surrounding the armature. The magnetic field created by the armature is either attracted or repelled by the surrounding magnets and thus induces a rotation of the armature. As the armature rotates, the attached commutator bars rotate with it, while the brushes remain stationary. As a result of the armature rotation, the brushes contact new commutator bars, electrical energy is applied to the successive set of windings, and the process of driving the armature in a given rotational direction continues.
Propulsion systems such as those described above, utilizing brush-type DC motors, have well served their intended use, but they are not without the possibility for improvement. For example, commutated motors require frictional contact between the commutator and brushes, resulting in mechanical and electrical noise. The inevitable wear of the brushes produces particulates which are detrimental to the motor""s performance, such as contamination and degradation of the bearings. Furthermore, because the electrical windings are located away from the housing, and because the majority of heat is generated by the windings, heat transfer is inefficient. Heat must travel through an air gap, through the magnets, and then through the housing to be dissipated. All of these factors lead to inefficiencies and/or shortening of the useful life of the motor.
Certain systems have been proposed that employ various types of brushless DC motors for such drives. However, these systems too have suffered from relatively low efficiencies, resulting in high energy consumption, excessive heating, and the need to oversize the motors to obtain the desired output thrust.
There is, therefore, a need in the art for a watercraft propulsion system which reduces the drawbacks of commutated DC motors, such as frictional contact within the electric motor, particulate production within the electric motor, and so forth. There is also a need for trolling motors and electric outboards which are more efficient, run cooler, and offer a smaller and lighter-weight overall package.
The invention provides a propulsion system designed to respond to these needs. In accordance with one aspect of the invention a propulsion unit for a watercraft is provided that includes a prop adapted for producing thrust in forward or reverse directions when rotated about a central axis. An electric motor is drivingly coupled to the prop. The electric motor is of the switched reluctance type which includes a stator, formed of a plurality of windings, and a rotor having a plurality of poles about the circumference of the rotor. The stator windings are divided into a plurality of phases which are energized by electric current during operation. A controller may be coupled to the electric motor to apply drive signals in accordance with the desired order and timing of phase energization. This control is then configured to respond to the torque, speed or direction requirements imposed on the motor by the watercraft.
In accordance with another aspect of the invention, a method is provided for propulsion of a watercraft. In the method, a prop is adapted for displacing water upon rotation of the prop about an axial centerline. The prop is drivingly coupled to an electric motor which has a stator and a rotor. The stator is divided into a plurality of phases and the rotor is provided with a plurality of poles about its circumference. A first phase of the stator is then energized with an electrical current. The first phase is then de-energized by withdrawing the electrical current. Upon de-energization of the first phase, a subsequent phase, adjacent to the first phase, is energized. The process of energization and de-energization continues, creating a dynamic magnetic field which has a pattern of rotation in a first direction (e.g. clockwise). The rotor is adapted to have the poles arranged such that the rotor rotates in response to the dynamic magnetic field.